1. Field of the Invention
This invention relates to tomosynthesis for obtaining a tomogram from x-rays, and more particularly to an x-ray system for obtaining a tomogram of the chest.
2. Description of the Related Art
In order to observe the diseased part, there has been carried out comparison between an x-ray image which was taken in the past and an x-ray image which was currently taken. Upon comparison between a past x-ray image (an x-ray image which was taken in the past) and a current x-ray image (an x-ray image which was currently taken), it is not easy to grasp the difference between the past x-ray image and the current x-ray image, for instance, when the past x-ray image was taken during shrinkage of the heart and the current x-ray image was taken during expansion of the heart since a state where the lung is inflated and a state where the lung is compressed and deformed are compared with each other. Accordingly, it has been proposed to detect the cycle of the heartbeat by measuring shrinkage and expansion of the heart by the use of an electrocardiograph, a sphygmograph or the like and take an x-ray image constantly during shrinkage of the heart. (See, for instance, Japanese Unexamined Patent Publication No. 2003-250790)
Further, tomosynthesis where the x-ray tube is moved to take x-ray images at different angles and the obtained images are added to obtain an image where a desired cross-section is enhanced is recently proposed in the CR (computed radiography) in order to observe the diseased part in more detail.
In the tomosynthesis, the x-ray tube is moved in parallel to a flat panel or along a circular or elliptical path to take x-ray images at different angles and the x-ray images are reconstructed to obtain a tomogram. A method of reconstructing the x-ray images to obtain a tomogram will be described, hereinbelow.
When the x-ray tube is moved to positions S1, S2, . . . Sn to take images of an object body 5 at different angles, different x-ray images I1, I2, . . . . In are obtained as shown in FIG. 1. When, for instance, x-rays are projected from the position S1 of the x-ray source onto two object parts (01 and 02) at different depths, images of the object parts (01, 02) are formed in positions P11 and P12 of the x-ray images I1, whereas, when x-rays are projected from the position S2 of the x-ray source onto the object parts (01 and 02), images of the object parts (01, 02) are formed in positions P21 and P22 of the x-ray images I2. When x-rays are repeatedly projected from the different positions of the x-ray source S1, S2, . . . Sn onto the object parts (01 and 02) in this manner, images of the object parts (01) are formed in positions P11, P21, . . . Pn1, and images of the object parts (02) are formed in positions P12, P22, . . . Pn2.
When the cross-section where the object part 01 exists is to be enhanced, the x-ray image I2 is shifted by a distance equal to the distance between P21 and P11, the x-ray image I3 is shifted by a distance equal to the distance between P31 and P11, . . . and the x-ray image In is shifted by a distance equal to the distance between Pn1 and P11, and the images thus obtained are added up, whereby a tomogram where the structure on the cross-section at the depth of the object part 01 is enhanced is created. When the cross-section where the object part 02 exists is to be enhanced, the x-ray image I2 is shifted by a distance equal to the distance between P22 and P12, the x-ray image I3 is shifted by a distance equal to the distance between P32 and P12, . . . and the x-ray image In is shifted by a distance equal to the distance between Pn2 and P12, and the images thus obtained are added up. Thus by adding up the images obtained by shifting the x-ray images I1, I2, . . . . In to be aligned with each other depending on the position of a desired cross-section, an image where a tomogram in a desired position is enhanced can be obtained.
By observing a tomogram obtained by the tomosynthesis, more detailed information can be obtained. However, when a tomogram of the chest is obtained by taking an image of the chest by the tomosynthesis, a light shadow such as of lung cancer sometimes does not appear in the tomogram by a motion artifact due to the heartbeat or the breathing.
When the heartbeat is detected and x-ray images are taken at the same period at the timing of shrinkage of the heart as in Japanese Unexamined Patent Publication No. 2003-250790, though the comparison between the past x-ray image and the current x-ray image is facilitated, the accuracy at which a light shadow such as of lung cancer can be recognized cannot be improved.